1. Field of the Invention
The present invention relates generally to implantable medical devices and to a method for manufacturing implantable medical devices. These implantable medical devices may also be capable of retaining therapeutic materials and dispensing the therapeutic materials to a desired location of a patient's body. More particularly, the present invention relates to a method for forming the structure of a stent or intravascular or intraductal medical device.
2. General Background and State of the Art
In a typical percutaneous transluminal coronary angioplasty (PTCA) for compressing lesion plaque against the artery wall to dilate the artery lumen, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced through the vasculature until the distal end is in the ostium. A dilatation catheter having a balloon on the distal end is introduced through the catheter. The catheter is first advanced into the patient's coronary vasculature until the dilatation balloon is properly positioned across the lesion.
Once in position across the lesion, a flexible, expandable, preformed balloon is inflated to a predetermined size at relatively high pressures to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile, so that the dilatation catheter can be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery. While this procedure is typical, it is not the only method used in angioplasty.
In angioplasty procedures of the kind referenced above, restenosis of the artery often develops which may require another angioplasty procedure, a surgical bypass operation, or some method of repairing or strengthening the area. To reduce the likelihood of the development of restenosis and strengthen the area, a physician can implant an intravascular prosthesis, typically called a stent, for maintaining vascular patency. In general, stents are small, cylindrical devices whose structure serves to create or maintain an unobstructed opening within a lumen. The stents are typically made of, for example, stainless steel, nitinol, or other materials and are delivered to the target site via a balloon catheter. Although the stents are effective in opening the stenotic lumen, the foreign material and structure of the stents themselves may exacerbate the occurrence of restenosis or thrombosis.
A variety of devices are known in the art for use as stents, including expandable tubular members, in a variety of patterns, that are able to be crimped onto a balloon catheter, and expanded after being positioned intraluminally on the balloon catheter, and that retain their expanded form. Typically, the stent is loaded and crimped onto the balloon portion of the catheter, and advanced to a location inside the artery at the lesion. The stent is then expanded to a larger diameter, by the balloon portion of the catheter, to implant the stent in the artery at the lesion. Typical stents and stent delivery systems are more fully disclosed in U.S. Pat. No. 5,514,154 (Lau et al.), U.S. Pat. No. 5,507,768 (Lau et al.), and U.S. Pat. No. 5,569,295 (Lam et al.).
Stents are commonly designed for long-term implantation within the body lumen. Some stents are designed for non-permanent implantation within the body lumen. By way of example, several stent devices and methods can be found in commonly assigned and common owned U.S. Pat. No. 5,002,560 (Machold et al.), U.S. Pat. No. 5,180,368 (Garrison), and U.S. Pat. No. 5,263,963 (Garrison et al.).
Intravascular or intraductal implantation of a stent generally involves advancing the stent on a balloon catheter or a similar device to the designated vessel/duct site, properly positioning the stent at the vessel/duct site, and deploying the stent by inflating the balloon which then expands the stent radially against the wall of the vessel/duct. Proper positioning of the stent requires precise placement of the stent at the vessel/duct site to be treated. Visualizing the position and expansion of the stent within a vessel/duct area is usually done using a fluoroscopic or x-ray imaging system.
Although PTCA and related procedures aid in alleviating intraluminal constrictions, such constrictions or blockages reoccur in many cases. The cause of these recurring obstructions, termed restenosis, is due to the body's immune system responding to the trauma of the surgical procedure. As a result, the PTCA procedure may need to be repeated to repair the damaged lumen.
In addition to providing physical support to passageways, stents are also used to carry therapeutic substances for local delivery of the substances to the damaged vasculature. For example, anticoagulants, antiplatelets, and cytostatic agents are substances commonly delivered from stents and are used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. The therapeutic substances are typically either impregnated into the stent or carried in a polymer that coats the stent. The therapeutic substances are released from the stent or polymer once it has been implanted in the vessel.
In the past, stents have been manufactured in a variety of manners, including cutting a pattern into a tube that is then finished to form the stent. The pattern can be cut into the tube using various methods known in the art, including using a laser.
Laser cutting of the stent pattern initially utilized lasers such as the Nd:YAG laser, configured either at its fundamental mode and frequency, or where the frequency of the laser light was doubled, tripled, or even quadrupled to give a light beam having a desired characteristic to ensure faster and cleaner cuts.
Recently, lasers other than conventional Nd:YAG lasers have been used, such as diode-pumped solid-state lasers that operate in the short pulse pico-second and femto-second domains. These lasers provide improved cutting accuracy, but cut more slowly than conventional lasers such as the long pulse Nd:YAG laser.
Throughout the process of fabricating a stent implant from raw tubing there is a general desire to minimize the amount of contamination in non-beneficial materials affects that can result from the introduction of high heat and formed substances to the tubing. One process that is particularly susceptible to these affects is the laser cutting process, since it introduces both heat in the form of laser energy and foreign materials in the form of shielding gases and surrounding environmental gases into the stent cutting process. Oxygen is a particular concern because it can lead to material oxidation and embrittlement of the material due to reactions between the oxygen and the tubing material in the presence of the heat generated by the laser-cutting beam.
A further concern is the affect of molten stent material and ablated debris generated during the laser cutting process. As tubing is melted or ablated by the laser beam to form a stent structure, the shielding gas, which is typically an inert gas such as argon, directs molten material away from the raw tubing. At least a portion of this material is ejected in a direction that generally opposes the direction of the laser beam. Under certain conditions, such as when the shielding gas is flowing at a low rate, or when the shielding gas nozzle is wide enough to allow entry of particulates, the laser optics can become marred by the escaping particulates. The particulates may deposit on the lens of the laser equipment and over time, these depositions can obscure the path of the laser beam creating detrimental changes the laser beam characteristics. For example, the beam may lose focus, which can result in a less clean cut or longer cutting times. Another concern is the debris from laser cutting may be accumulated inside the cutting kerf, thereby reducing the cutting efficiency of the laser beam.
Another problem with using a laser to cut a stent pattern into a tube is that to ensure that a laser will cut the tubing used to form stents, there must be appropriate laser power to melt or ablate the tubing material. However, when the laser cuts through the tubing, the laser beam may propagate beyond the tubing wall and may melt or burn the opposing wall of the tubing material. This may cause defects in the stent pattern which is ultimately cut from the opposing wall of the tubing.
What has been needed, and heretofore unavailable, is an efficient and cost-effective laser cutting system that incorporates various features designed to enhance the cutting performance of the laser while protecting both the laser and the manufactured article from detrimental effects due to the presence of an undesired environmental gas, such as oxygen and damage to the laser optics and article due to particulates generated from the material being cut. Such a system and method would also provide for preventing material and laser optic contamination throughout the laser cutting process by ensuring that molten and material and ablated debris other resultant substances are optimally withdrawn from the cutting zone of the laser cutting equipment. Further, such a system should be capable of cutting a stent pattern into a tubing wall while avoiding damage to the opposing wall of the tubing. The present invention satisfies these, and other needs.